Nerve localization (NL) is a common procedure in the application of regional anesthesia and in the localized treatment of certain neuropathies. Typically, the physician passes a needle through the skin and subcutaneous tissue of the patient, and then advances the needle into close proximity to a target nerve, at which point a pharmacologically-active agent (e.g., corticosteroids, lidocaine, etc.) is delivered to the nerve.
One of the clinical challenges associated with NL is the need to position the needle within close proximity to the target nerve, but without physical contact.
Electrical nerve stimulation (NS) may be used to help guide needle movement during a NL procedure. More particularly, after the needle is inserted in the patient, an electrical stimulator delivers an electrical current through the needle that is high enough (typically a few milliamps) to stimulate the nerve so that a visible muscle contraction response (for a motor nerve) or a patient-detectable sensory response (for a sensory nerve) is obtained. The magnitude of the current is thereafter manipulated (generally decreased) as the needle is being advanced. When targeted muscle contractions are visually observed (or when sensory responses are being reported by the patient) at a low pre-determined current magnitude (typically 0.2-0.5 milliamps), it is believed that the tip of the needle is sufficiently close to the nerve for effective delivery of the pharmacologically-active agent.
While the prior art electrically-guided nerve localization procedure discussed above may be an improvement over “blind” nerve localization, it is nonetheless limited for a number of reasons.
1. The assessment of a motor response to the electrical stimulation, which is critical to judging the proximity of the needle to a target motor nerve, is based solely on a subjective visual observation of muscle contraction by the user (e.g., a physician or other healthcare provider). Quantitative and objective measures of motor response intensity (i.e., measures that do not rely on a visual impression) are not utilized in current clinical techniques. The stimulation threshold for an observable muscle contraction is higher than that of an electrically recordable response (i.e., a compound muscle action potential, also known as a CMAP) and therefore current methods are not optimally sensitive.
2. The assessment of a sensory response to the electrical stimulation, which is critical to judging the proximity of the needle to a target sensory nerve, is based solely on the report of the patient. Again, quantitative and objective measures of sensory response are not utilized in current clinical techniques. Among other things, if the patient has a diminished capability of reporting sensory response (i.e., nerve sensations, or “feeling”) because of neuropathy and/or because of sedation, the utility of a conventional nerve stimulator can be highly compromised. This is because a patient's sensory nerve may respond but the patient may not be able to sense it.
3. With prior art electrically-guided nerve localization, the user (e.g., a physician) must manually adjust the stimulation current intensity (usually amplitude, but the user may also adjust duration) while trying to delicately advance the needle and observe the response. This often requires two healthcare providers to perform such a procedure.
4. With the current clinical approach, it is generally concluded that the appropriate needle approximation has been achieved when the stimulation current has been reduced to some low pre-determined current magnitude, e.g., for anesthetic or drug injection, the target current is typically 0.2-0.5 milliamps (mA). However, the target current commonly varies from case to case. Thus, there is a general lack of precision and consistency with the current clinical approach.
5. In some situations the nerve stimulator response may not provide useful information and may even provide misleading information, e.g., when the target nerve has conduction problems and the motor/sensory responses are abnormal. More particularly, patients typically do not undergo a neuropathy diagnosis prior to the NL procedure. When the nerve responses are weaker than normal, a higher stimulation current intensity is required in order to evoke the same nerve response level for the same needle-nerve proximity. In this case, when the needle is already in the desired position for injection, the stimulation current may be still higher than the pre-determined threshold (0.2-0.5 mA) that is appropriate for a healthy nerve but may not be appropriate for the nerve that has neuropathology. As a consequence, the operator may not believe that the needle is close enough to the target nerve and therefore advance the needle even further, which can result in nerve injury.
6. The nerve stimulators currently in use do not have the capability of logging data and tracking the outcome of the procedure, which can be important for both clinical and medicolegal purposes.
7. Current nerve stimulators do not have the capability of permanently storing the data.